1. Field of the Invention The present invention relates to a method for manufacturing a liquid crystal display (or "LCD") device including a thin film transistor (or "TFT"). More particularly, the present invention relates to a method for etching a panel of an LCD in which the end point of the etching period is exactly detected so that an etched layer is not over-etched.
2. Discussion of Related Art
Referring to FIG. 1, showing a conventional LCD arrangement, and FIG. 2, showing the cross sectional view along with the reference line II--II in FIG. 1, the conventional LCD comprises a plurality of gate lines 70 on a substrate 10 and a plurality of data lines 80. The gate lines 70 and the data lines 80 intersect each other in a substantially perpendicular arrangement to form an LCD matrix. The LCD further comprises a pixel electrode 40 at each area in the matrix surrounded by two neighboring gate lines 70 and two neighboring data lines 80. A TFT 31 is provided connected to the pixel electrode 40 at the intersection portion of one gate line 70 and one data line 80. An overlapped portion of the pixel electrode 40 and the gate line 70 is a storage capacitance electrode 35.
The TFT 31 comprises a gate electrode 70a extruded from the gate line 70, a source electrode 80a extruded from the data line 80, and a drain electrode 80b which faces the source electrode 80a. The TFT 31 further comprises a semiconductor layer 90 over the gate electrode 70a on a gate insulation layer 50 covering the gate line 70 and the gate electrode 70a. The semiconductor layer 90 is in contact with the source electrode 80a and the drain electrode 80b. In order to ohmically contact the source 80a and the drain 80b electrodes with the semiconductor layer 90, doped semiconductor layers 92a and 92b are formed under the source electrode 80a and the drain electrode 80b, respectively.
The LCD also includes a protection layer 55 covering the whole surface of the substrate 10, which includes a TFT 31, a contact hole 37 which exposes a portion of the drain electrode 80b by etching the protection layer 55, and the pixel electrode 40 coupled to the drain electrode 80b through the contact hole 37. If the duration of the etching process on the protection layer 55 when the contact hole 37 is formed lasts longer than a desired interval, then the resulting contact hole is larger than the initially desired size. As a result, the pixel electrode 40 does not cover the contact hole 37 entirely and leaves exposed a portion of the drain electrode 80b. This situation allows the exposed portion of the drain electrode 80b to be in contact with moisture and oxygen in the atmosphere. As a result, the drain electrode 80b can be corroded and short circuited, as shown in FIG. 2.
In order to understand why the contact hole is over-etched, we will explain the processing for manufacturing the LCD in detail with reference to FIGS. 3A and 3B.
As seen in FIG. 3A, a gate electrode 70a including aluminum or an aluminum alloy is formed on a transparent substrate 10. A gate insulating layer 50 including a silicon nitride (SiN.sub.x) or a silicon oxide (SiO.sub.x) is deposited on the substrate 10 covering the gate electrode 70a. A semiconductor layer 90 is formed on the gate insulation layer 50 over the gate electrode 70a. Two doped semiconductor layers 92a and 92b are formed on the two sides of the semiconductor layer 90. A data line 80 including a chromium material is formed on the gate insulation layer 50. A source electrode 80a extruding from the data line 80 is formed on the doped semiconductor layer 92a. A drain electrode 80b facing the source electrode 80a is formed on the doped semiconductor layer 92b. After the TFT including the source 80a and drain 80b electrode is formed, a protection layer 55 including an Si bonding structure such as a silicon nitride, a silicon oxide, or a benzocyclobutene (or "BCB") is formed on the substrate including the TFT. A photoresist is coated on the protection layer 55 using a spin coating method. The photoresist is exposed and developed using a mask to form a pattern layer 60 in which a portion having a scale d1 is removed from the photo resist.
After the panel of the LCD, as seen in FIG. 3A, is placed in an etching chamber, the exposed portion of the protection layer 55 is removed using SF.sub.6 gas as an etchant. When the etchant is in contact with the surface of the protection layer 55, the SF.sub.6 gas reacts with the Si radical of the protection later 55, so that the exposed portion of the protection layer 55 is removed from the surface while producing a volatile material such as an SiF.sub.4 gas. After the exposed portion of the protection layer 55 is entirely removed, the surface of the drain electrode 80b is revealed. At that time, the amount of SiF.sub.4 gas produced is decreased conspicuously. In other words, when the SiF.sub.4 gas is being produced, the etching process is proceeding. The conspicuous reduction in the production of SiF.sub.4 gas is an indication that the etching process is ended. We can determine the time that the optimum etching duration end point is reached by monitoring the amount of the SiF.sub.4 gas produced.
However, despite the hundreds of thousands of the contact holes 37 present in the arrangement, the total area of the exposed portion of the protection layer is less than 1% of the total area of the panel, because the plane view area of the contact hole is less than tens of .mu.m.sup.2. Therefore, it is difficult to detect the etching duration end point exactly using the small amount of the SiF.sub.4 gas reproduced from this tiny area. For example, in this method for detecting the SiF.sub.4 gas, an electrical device, referred to as an etching end point detector, in which the amount of the SiF.sub.4 gas produced is converted to electrical voltage according to the time duration is used. As seen in FIG. 4, the difference between the indicated voltage (1) during the time that the etching process is proceeding and the indicated voltage (3) during the time that the etching process is finished is difficult to distinguish because of the relatively slight difference between the two voltage levels. This results in difficultly in determining the exact duration of the etching period and the proper end point for the etching. By not determining the exact desired end time of the etching process, generally, the etching process continues after the desired end time is passed, resulting in the protection layer 55 being over-etched so that the contact hole has a d2 scale instead of the desired d1 scale as shown in FIG. 3B.
At this time, the pattern layer 60 is removed and an indium tin oxide (or "ITO") layer is deposited on the protection layer 55. The ITO layer is patterned to form a pixel electrode 40. In this case, the drain electrode 80b is not entirely covered by the pixel electrode 40. The drain electrode 80b is exposed through the contact hole 37 enlarged by the over-etch, as seen in FIG. 2. The exposed portion of the drain electrode 80b contacts with moisture and the oxygen in the atmosphere. As a result, the drain electrode 80b can corrode and finally it can be opened to form a short circuit.
The foregoing discussion describes the over-etch problem of the contact hole formed at the protection layer. A similar problem may result when a metal layer is etched to form a gate or data line, or when a semiconductor layer is formed.